Liquid crystal materials for electro-optical indicating devices frequently contain one or more optically active additives for the induction of a chiral structure. For example, for use in indicating devices having a twisted nematic structure a nematic liquid is preferably doped with an optically .active additive, e.g. in order to avoid a reversal of the twisting direction (reverse twist) in TN cells (twisted-nematic) or in order to achieve an adequate twisting in cells having a highly twisted nematic structure such as STN cells (super twisted-nematic), SBE cells (super birefringence effect) or OMI cells (optical mode interference). Further, cholesteric liquid crystals for phase-change cells can preferably consist of a nematic basic material and one or more optically active dopants and ferroelectric liquid crystals for indicating devices based on chiral tilted smectic phases can preferably consist of a material having a tilted smectic phase and one or more optically active dopants.
The electro-optical characteristics of liquid crystal indicating devices are temperature-dependent, which is especially troublesome in the case of multiplex operation. It is, however, known that this temperature dependence can be compensated at least partially by adding chiral dopants which induce a decreasing pitch with increasing temperature. Such an inverse temperature dependence has been found only for a few compounds. It can, however, also be achieved by using at least two chiral dopants which have a different relative temperature dependence and which induce a different twisting direction as described in U.S. Pat. No. 4,264,148 corresponding to DE-A-2827471. Of course, this usually requires a relatively high amount of chiral dopants.
Cholesteric liquid crystals reflect light essentially only in a wavelength range for which the wavelength is about the same as the helical pitch. The spectral width of this reflection light can be varied by suitable choice of the liquid crystal. The reflected light is completely circularly polarized. The direction of rotation of the reflected light depends on the direction of rotation of the cholesteric helical structure. The opposite circularly polarized light is transmitted unimpaired. These properties can be employed for the production of optical filters, polarizers, analyzers etc. Further, cholesteric liquid crystals have also variously been used for thermochromic applications and in cosmetic preparations.
Cholesteric liquid crystals for the above applications preferably consist of a nematic or cholesteric basic material and one or more chiral dopants, which permits a simple adjustment of the desired helical pitch.
In order to achieve cholesteric mixtures having a pitch in the range of the wavelength of visible light, the chiral dopants should have a twisting capacity which is as high as possible and should have a good solubility in usual liquid crystal materials. Furthermore, the chiral dopants should have an adequate stability, should have a good compatibility with the mesophase type of the liquid crystal material and should not restrict the mesophase range too strongly. Such properties would also be desirable for chiral dopants for producing the twisted nematic structures referred to earlier, since their amount can be held low in order that the properties of the liquid crystal material are influenced only immaterially.